[Q] Waiting for devices (While rooting phone)

Hi All...I am a noob when it comes to smartphones. I am trying to root my phone(Nexus 5) for the first time and whenever I open up the command window and hit any key to continue. It stuck to waiting for device...

In the command window it says that after the unlovk you should see a big red android sign but nothing shows up....The same green icon appears stating the Phone status as lock at the bottom.

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